1. Field of the Invention
The present invention relates to a differential interference microscope apparatus employed for observing, e.g., a living specimen. The present invention also relates to an observing method using the differential interference microscope apparatus.
2. Related Background Art
A conventional differential interference microscope includes, e.g., a transmitted illumination type of microscope as shown in FIGS. 5A and 5B. This type of differential interference microscope is constructed as follows. Referring to FIG. 5A, a specimen M is Koehler-illuminated with a beam of light from a light source S through lenses L.sub.1, L.sub.2 and a condenser lens L.sub.3. The transmitted light of this specimen M forms a magnified image Y via an objective lens L.sub.4. The magnified image Y is observed with a naked eye through an eyepiece L.sub.5 in a microscope optical system. This microscope optical system incorporates differential interference members, i.e., a polarizer P, an analyzer A, a Wollaston prism W.sub.1 and a Wollaston prism W.sub.2.
The Wollaston prism W.sub.1 is disposed in a position of entrance pupil of the condenser lens L.sub.3. The Wollaston prism W.sub.2 is disposed in a position of exit pupil of the objective lens L.sub.4. The polarizer P is disposed on this side (on the side of the light source S) of the Wollaston prism W.sub.1. The analyzer A is disposed in rear (on the side of the eyepiece L.sub.5) of the Wollaston prism W.sub.2.
The Wollaston prism is, as illustrated in FIG. 6, constructed by bonding two pieces of right-angle prisms cut out in a state where directions of their optical axes are orthogonal to each other. Herein, the directions (shown by arrowheads) of one optical axis are set parallel to the sheet surface, while the directions (marked with (+)) of the other optical axis are set perpendicular to the sheet surface. This prism separates a beam of incident light into two beams of rectilinear polarized light having oscillatory planes orthogonal to each other. At this time, the beam of incident light is separated into the two beams of polarized light at an angle-of-deviation .theta. determined by an apical angle .alpha. of the right-angle prism. The angle-of-deviation .theta. is given by: .theta.=2(n.sub.e -n.sub.0) tan.alpha., where n.sub.e is the refractive index of the Wollaston prism with respect to an extraordinary ray, and n.sub.0 is the refractive index of the Wollaston prism with respect to an ordinary ray.
According to the above construction, as illustrated in FIG. 5B, the rectilinear polarized light in the arrowed direction is taken out of the light emerging from the light source S by means of the polarizer P. This rectilinear polarized light is separated by the Wollaston prism W.sub.1 at the angle-of-deviation .theta. into the two beams of rectilinear polarized light, which are orthogonal to each other. Normally, the Wollaston prism is selected corresponding to an objective lens employed but is replaced together when exchanging the objective lens.
The thus separated two light beams fall on the condenser lens L.sub.3. If the Wollaston prism W.sub.1 is located on a front-side focal plane of the condenser lens L.sub.3 at this time, however, the two beams of rectilinear polarized light become parallel beams spaced by a quantity (shear quantity) S away from each other. The shear quantity S is determined by the focal length f.sub.c of the condenser lens L.sub.3 and the angle .theta. but is given by S=f.sub.c .multidot.tan.theta.. The specimen M is then irradiated with the parallel beams.
The two light beams penetrating the specimen M converge on a rear-side focal plane of the objective lens L.sub.4 via the objective lens L.sub.4 and become a single beam of light through the Wollaston prism W.sub.2 disposed therein. This beam of light travels on the same optical path. With a further transmission through the analyzer A, antiphase components of these two light beams are taken out, resulting an interference with each other.
That is, if there is produced no difference in terms of the optical path between the two light beams due to a penetration into the specimen M, the two light beams interfere with each other, offset each other and darken. Whereas if the difference in the optical path is produced, however, the beams look bright. The differential interference microscope utilizes this principle. Even when the specimen M is colorless and transparent, but if there is caused the difference in the optical path between the two light beams in accordance with a difference in terms of a thickness or a refractive index within the specimen M, the specimen M can be observed with a difference in brightness.
In this type of differential interference microscope, the specimen is observed mainly with the naked eye, and therefore, the image Y is required to have a contrast to some extent. This contrast is determined by a magnitude of the shear quantity S. When enhancing the contrast, the shear quantity S may be increased. Supposing that, for instance, as illustrated in FIG. 7A, a portion M' exhibiting a high refractive index exists in the specimen M, and when the same phasic plane of the beam is expressed by a line segment ab, the incident light ab, after passing through the specimen M, travels forward taking a shape a'b'. At this time, the two light beams spaced by the shear quantity S are overlapped with each other as shown in FIG. 7B. The contrast is, however, obtained with brightness and darkness, which are produced due to an optical path difference .DELTA. between the two light beams. As obvious from the Figure, when the shear quantity S is small, the optical path difference .DELTA. between the two light beams results in a lower contrast. When the shear quantity S is taken large, the contrast is enhanced. The contrast between the brightness and darkness is produced even with a slight inclination.
In the above-described conventional differential interference microscope, however, when enhancing the contrast by increasing the shear quantity S, a resolving power of the objective lens L.sub.4 has to be sacrificed to some extent.
For example, when the shear quantity S exceeds the resolving power .delta. (.delta.=0.61.times..lambda./N.A.) of the objective lens, the image Y of the specimen M looks double. Further, even when the shear quantity S is not so large but set under the resolving power .delta. of the objective lens L.sub.4, and if set in the vicinity of the resolving power .delta., there happens such a phenomenon that the image is extended in the direction of this shear quantity S.
This conduces to such a problem that hyperfine portions appear with no contrast, which are to be originally resolvable by a performance of the objective lens. Hence, it has hitherto been desirable that the shear quantity S be decreased so as not to spoil the resolving power of the objective lens to the greatest possible degree. For obtaining a relatively high contrast, however, there is a limit wherein the shear quantity S is set on the order of .delta./2.